Synchronizing system



Nov. 8, 1949 K. R. WENDT SYNCHRONIZING SYSTEM 3 Sheets-Sheet l Filed Jan. 18, 1946 ATTORNEY LEM"- Naw., gp

K. WENDT SYNCHRONIZ ING SYSTEM Filed Jan. 18,' 1946 wo anu/N5 (B) feo/w64) 5MM/ave (c) M M )52 CNP/Dio faz/55 (D) xfa/vnu af;

3 Sheets-Sheet 2 fifi/V67@ INVENTOR Karl Ulend ATTORNEY NW 3%@ K. F2. www www SYNCHRONIZING SYSTEM E Sheets-Sheet I5 Filed Jan. 18, 1946 Patented Nov. 8, 1949 varl R.. `I'JI'ightstow'n;Ng J., assignor t0 v'Radio Corporati'in of America, a corporation of Delaware .Applicationinaryla 1916,`s'ria1No.'642,46

missien-systems Vand `m0,ref particulallv te Vilall- Dr, ed methoa ci and@.Ingallsv for :synchronizing urality ofjwave generators.

. The invention, fby Wav `f exanpl winnende ,scribedherenaten as improvement vin a Secret telecommunicatign system ofthetvpe deed'in the @pending U-S. applicatiornof H Y Bedf9rd, Seria1 No. 536,630 ledMay 2Q, 1944 nowfjU.' `S.'1 Patent-No. 2,401,405, granted ,JuneyfL 194,635 Said copending, application dis: closesandelaimsa synchronizing system forja secret sighallingcsystem wherein, for example, a speechg signal' comprising a complex Wave S is modedbytmeans cfa coding signal. comprising 4a complex wave K in a. manner whereby the instantmeous` ordinates :of f,the resulting coded `signals arethe product SK of the corresponding` in stantaneous ordinates of the speechlsign'al and the, codingffsignal.' The Vresulting unintelligible coded signals are transmitted by `any conventional 4means to areceiver whereinthe coded signals,` are .combined with decodingsignals generated in the receiver Aand` having instantaneous ordinates cor.- responding totheireciprocalsof the corresponding-instantaneous ordinates of fthe `coding signal -componentvof :the :transmitted signal.. r .The 'ina'l signals,ithereforerarederived from thproduct of the ytransn'iitted signaLSK and they. decoding. sig

nalfj/K.. i

i `'Iyhe systernzdisclosedtherein 'includes lairneth'- odrof zand means nfor synchronizing' at `"predetermined interva'lsi'the codingta'rid decoding @Wave generators 'lbymeanslfof speciali4 synchronizing pulsesignal'sach; comprisinga first signalJpulse "immediatelyY followedrby` aesecondgsignal upulselof oppositeipolarity; which pulses may be superimposed AuponA the coded signals SK.l At thereceiverfithe signals are differentiated, and the 'reversalA inl polarity between Y the two synchronizing pulses `isemployed 'to synchronize the. `de` coding wave' generator. l Y, f` A'Id-isadvantage" of the system disclosed Vinsaicl lcoper-idingvapplication is Vthat the synchronism -of -th'e receiver coding Wave generator VYis i dependent upon pulses derived-by-diierentiation of the received signal.v Thereceiver codinglwavengere- -erator` is freeerunningrand thus. may iba-'acme ated by ,more for: less "recurrent noiseLsignals or large,variations in rateof magntudeyottheconn `n,lilrlationsienak 1 f The instant invention: comprises fari improves? 'ethod of and means for insuringucloseandrapid 4fflifronisinof t ,transmitter and receiver hcod,- 'ing wave generators aidhighimmunity wn is'e 13 clans; (c1. 17a-69.5)

forwother signals extraneous to the transmitted `synchronizing pulses. Noise and extraneous signal immunity is provided by making the receivensynchronizing system inoperative during the major portion' of the signal period and selec- Y'tive'lyoperative onlyiduring an interval beginning about seven electrical' degrees before and lasting until about three degrees after the transmitted synchronizing pulses are due to arrive at the receiver: The synchronizing circuit of Ythe receiver thusisn receptive to" received signals Thaving aA high rate 'ofchange of magnitude only during 1/36 of; thesignalling period. Theaccurate Vtiming is provided' by means of the 'receiver delay line" which furnishesrecurrent gating :pulses to' activatethe 'synchronizing' circuits for` operation by the received d iierentiated synchro'- xnizinglpu'lses." Variable phasing or 'timing of the activationinterval of the receiver synchronizing 'circuit is` provided by a variable Width multi'- j/ikjrator;`

'P :Lvgflief'frontjedggef`of the 'multivibrator pulse is triggered bythe receives differentiated synchro'- Iiiz'ing's'ignal and the` back edge of"`the pulse initiates the keyig'of the delayline.' Provision als'ois'made for'la variable delay period between thef'ccurrence f'the pulse' derived from the end ofthe delay line and'thegating time of the multiviratorff This. accomplished by making the 'totalperating time of two.`multivibrators a lconstant time interval of l'f the pulse period, as will be describedinl-greater detail hereinafter. variable width multivibrator pulse intervals determined by@J `common phasing circuit for o two lmultivibrators. A blanking multivi- `-bratoractuated by pulses from the end of the ,delay line blanks the communication signal during the synchronizing intervals and supplies A ctivating pulsesior gating the synchronizing A innltii/ ihrators. V V ,t

,zj'It should be understood `that while the present inventionV is described with'reference to thetype `o f system disc'losedffin said copending applica- 4tionfit; also may-beemployed for synchronizing anyy "known, types of i Wavegenerators wherein synchronizing wave control pulses` may be emfployed,A and-wherein relatively precise synchronism is desirable; f f 'I Among tljieobjects, ofthe invention are to provide lanimproved methodof and means for synchronizinga plurality o f Wave generators. Anothenobject onthe invention 4is to provide an improved methodlcf and means for synchronizing, morepreeiselyv thanfwas heretofore possible, wavesi generated by `a plurality of separately 3 pulse-excited delay networks. Another object of the invention is to provide an improved method of and means for synchronizing secret telecommunication systems. An additional object is to provide an improved synchronizing pulse generator. A further object is to provide an improved receiving network for synchronizing a local wave generator with a remote wave generator in response to synchronizing signals combined with signals received from said remote wave generator, and wherein the receiver is recurrently and selectively responsive to said synchronizing signals only during a predetermined portion of each synchronizing pulse period. Another object is to provide an improved synchronizing system wherein synchronizing signals comprising a rst pulse immediately followed by a second pulse of opposite polarity are transmitted to a receiver responsive to the reversal in polarity between said pulses. An additional object is to provide an improved synchronizing system for a plurality of wave generators responsive to the reversal in polarity between two successive synchronizing pulses and providing tight lock-in, rapid synchronization and limited but smooth phase adjustment between synchronizing and synchronized signals. Another object is to provide `a synchronizing system for a wave signalling system wherein synchronism is substantially independent of the character of communication or noise signals.

The invention will be described in greater detail by reference to the accompanying drawings of which Figure 1 is a schematic block circuit diagram of a complete secret telecommunication system employing a preferred embodiment of the invention, Figure 2 is a series of graphs illustrating the circuit operation of the transmitting portion of the system shown in Figure 1, Figure 3 is a series of graphs illustrating the operation of L the receiver portion of the circuit illustrated in Figure 1, Figure 4 is a schematic circuit diagram of the synchronizing circuit according to the invention and forming a portion of the system circuit of Fig. 1, Figure 5 is a schematic circuit diagram of the wave multiplier forming a portion of the circuit of Figure 1, and Figure 6 is a schematic, circuit diagram of a wave reciprocal circuit forming a portion of the system in Fig. l. Similar reference characters are applied to similar elements throughout the drawing.

Coding wave generator Referring to Figure 1, the coding wave generator employed for both transmitting and receiving coded speech signals includes a conventional freerunning multivibrator circuit l which generates pulses at a rate, for example, of one hundred pulses per second. A typical multivibrator of this type, the frequency of which may be controlled by recurrent applied control pulses, is described in U. S. Patent 2,266,526, granted to E. L. C. White on December 16, 1941. It should be understood that pulses of either polarity may be applied in any known manner to key the multivibrator, and that, similarly, output pulses of either polarity may be derived therefrom. The generated pulses are applied to the input of a differentiator-amplifier circuit 3 which supplies short sharp pulses Vto a conventional delay network 2 comprising a plurality of series inductors 5, 1, 9, II, I3 and a plurality of shunt capacitors 4, 6, 8, IIJ, I2, I4. The remote terminals of the resultant pulse delay network 2 are terminated by a resistor I5 matching the surge impedance of the network. It should be understood that the delay network 2 may include a relatively large number of filter sections as indicated by the dash lines interconnecting the filter sections 8, 9 and I0, II, and that equalizers and booster ampliiiers may be inserted in the delay network at desired points to maintain pulse amplitude relations at optimum values.

Pulses applied by the multivibrator i and differentiator-amplier 3 to the input of the delay network 2 provide similar pulses at the junction kof each of the succeeding series inductors 5, 1, 9,

II, I3 wherein each succeeding pulse is delayed a predetermined amount with respect to pulses occurring at other prior network terminals. A complex coding wave thus may be obtained in response to each pulse applied to the delay network by combining in either polarity differently delayed pulses derived from a plurality of such predetermined points along the delay network.

Separate isolating resistors I1, I9, 2l, 23, 25, 21 each have one terminal connected to different points along the delay network, and have their remaining terminals connected to separate movable contacts of a plurality of single-pole doublethrow switches 29, 3|, 33, 35, 31, 39. VThe corresponding iixed contacts of the several switches are connected together to provide two lines 4 I, 43, which are terminated through resistors 45, 41, respectively, to ground. The remaining terminal of the line 4I is connected through a coupling resistor 49 to one xed contact T1 of a rst transmit-receive single-pole, double-throw switch 5I. The remaining terminal of the second line 43 is connected through a polarity-reversing amplifier 53 and a second coupling resistor 55 to said rst fixed contact T1 of the rst transmit-receive switch 5I. Thus each of the 100 pulses per second, derived from the multivibrator I and applied to the input of the delay network 2, provides a plurality of pulses of either polarity occurring at predetermined intervals during each one-hundredth second period, as determined by the points of connection to the delay network and the arrangement of the switches 29, 3l, 33, 35, 31, 39. Therefore, a very complex coding wave may be applied to the first fixed contact T1 of the rst transmit-receive switch 5I, merely by selecting the desired arrangement of the pulse selecting switches. It should be understood that the total delay provided by the pulse delay network should be at least slightly less than the pulse period of the multivibrator I in order that only one pulse may be traveling along the delay network at any predetermined instant.

In the typical secret telecommunication system described in said copending application identified heretofore, the coding signal generator includes a delay network having 80 sections and a plurality of sequential switches which may be pre- ,V set to any desired code and selectively actuated by a clock mechanism to change the code continuously or at predetermined desired intervals. Identical coding signal generators are employed at both the transmitter and receiver in such a secret telecommunication system, By means of simple transmit-receive switches the coding signal either is combined with the speech signal for transmitting a coded wave, or reciprocal values of the coding signal are derived from a re- "ciprocal circuit responsive t0 the coding signal lgenerator and are combined with the received apparatus. Hence, by means of the simple trans.

init-receive switches, the various elements of the `apparatus may be employed at different times for dual purposes in a single unit for either transmitting or receiving the coded signals.

be employed as a coding transmitter by switch- 4ing the movable contacts of each of the singlepole, double-throw transmit-receive, switches included therein to engage the fixed contacts T1, T2, T3, T4, T5, Ts, T7, Ts corresponding to the transmit condition. Signals derived, for example, from a microphone 57, which may befed lthrough a speech amplifier, not shown, are applied through asecond transmit-receive switch 59 to one input circuit of aV wave multiplier 6l,

which will be described in detail hereinafter by reference to Figure 5 of the drawings. Coding signals, from the coding signal generator described heretofore, are applied, through the rst switch 5l, to a second input circuit of said Wave multiplier Sl, whereby coded signals SK having instantaneous ordinates corresponding to the products of the corresponding instantaneous ordinates of the speech signal S and the coding signal K are applied through a third transmitreeive switch 63, a fourth switch 65, a signal blanking circuit 67 and a fth switch 69 to one input circuit of a first mixer circuit il, which may comprise any conventional network wherein applied signals are combined algebraically.

Transmitter synchronizing pulse generator Regularly recurrent pulse trains indicated by the graph A of Figure 2 are derived, for example, from the output circuit of the delay network 2 and applied to a conventional thermionie tube amplitude limiter circuit i3, which clips the wave A at the level p to derive individual limited pulses represented by graph B of Figure 2. The limited pulses B are applied to key a second or blanking multivibrator 15 to derive a negative blanking pulse illustrated by graph C of Figure 2. The negative blanking pulse C is applied to key the blanking circuit 61 to blank-out" the coded signals SK during the interval of the synchronizing pulses to be described hereinafter. The leading edge of the blanking pulse C keys a third multivibrator 7l to provide a negative pulse idealistically shown in graph D of Fig. 2. The pulse D is applied to a second input circuit of the mixer ll. The negative pulse D is also applied to key the first multivibrator l in response to the trailing edge of the pulse to provide a positive pulse which actuates the difierentiatoramplifier 3.

The positive pulse from the rst multivibrator is idealistically shown in graph E of Fig. 2. It

Will be understood that the positive pulse E will be initiated at the termination of the negative pulse D in a manner well known in the multivibrator art. The positive pulse E is applied to a third input circuit of the mixer circuit 'H 'whereby the coded signal SK, the negative pulse D and the positive pulse E are combined to provide a communication signal including the coded wave ASK and the synchronizing signal (as shown in `graph H of Fig. 2), comprising a negative pulse immediately followed by a positive pulse. It should be understood that, if desired, the syn- `chronizing signal maycomprise a positivepulse followed by a negative pulse since multivibrators may be keyed by, and can provide, pulsesfof .either polarity, providing. proper -connections interval.

`theret'oare.provided in a `maniienknown in the art. The .combined coded signal and synchronizing signal derived from the mixer 1| will have a waveform, for example, of the. type illustrated in graph IofFigure 2, including .the pulses J, J,

shown in dashlines.

The positive pulse derived from the rst multivibrator .I .also is appliedto the differentiator- `'amplier 3 to derive a positive keying pulse, .shown in graph F to key or excite the delay line2 to generate a succeeding wave train, shown in graph VGf of Figure 2, whichwill be progressively delayed along the delay network. Since .the rst multivibrator Vl is keyed by the pulse from the third multivibrator 11 immediately precedingthe' time-,fertile generation of a normal apulse bysaid first multivibrator, it will be seen thatthe coding wave generator will be self-running, and `will be maintained at a substantially :constant )frequencyfsince the pulse rate therethrough will be substantially dependent upon the'time delay ofthe successive pulses applied If for any reason the first multivibrator l is not properly keyed by the third multivibrator l1, the first multivibrator will merely generate a delayed pulse which will be applied to the delay network 2 at a slightly later The slightly delayed pulse upon reaching the last tap of th'e delaynetwork therefore willv key thesecond and thirdl multivibrators in the manner describedA heretofore and provide a 'new set of synchronizing pulses which will actuate. the first multivibrator I in synchronism thereafter. i

`The coded signals SK combined in the mixer circuit 1I with the-synchronizing pulses D and E are applied to aisecond limiter 83 whereby the high amplitude portions J ofthe synchronizing signal are clipped to maximum square wave levels Q, Q indicated by the dash lines in graph I of Figure 2. The thus' limited combined coded andlsquare wave synchronizing signals are applied as a communication signal to a conventional radio transmitter 85 which includes a transmittingv antenna 8T.

coding signez receiver` In orderv to convert the circuit thus described .to operate as a coded signal receiver, the movable contacts of each (of `the transmit-receive switches 5I, `lis, 63, 61, 69, 89, 9| and 93 are .switched to the corresponding fixed contacts R1, `R2, R3, R4, R5, Re, Rv, Rs corresponding to the .receive condition. VThe combined coded signal and synchronizing signals transmitted from the transmitter 85 are unavoidably smeared and phase-shifted somewhat in transmission to resemble the solid portion a: of the graph I of Figure 2, and asreceived by means of a conventional radio receiver 95, are applied to a wave limiter-differentiating network 91 which may be of vanytype well known in the art. For examfple, a wave may be differentiated by applying it to a network comprising a small series capacitor and a shunt resistor. The transmitted signal4 I of Figure 2 after being differentiated at the receiver resembles the graph L of Figure 3 wherein a relatively large pulse P occurs at an instant corresponding to the reversal in polarity between the .received synchronizing negative and i. positive pulses anad wherein low frequency components are'substantially removed from the pulse P. It should be understood that instead of differentiatingthe receivedsignal, it may be treated lin any other known manner to derive a Vpulsein vresponse to the reversal in polarityvof the negative and positive synchronizing pulses.

The receiver first multivibrator l being free running, as described heretofore, the delay network 2 Will provide recurrent pulses at its last tap which will be limited by means of the first limiter 'i3 to provide pulses for actuating the second multivibrator i5 for generating the receiver blanking signal represented by the graph M of Figure 3. The relatively longer receiving blanking pulse M is applied to the blanking circuit 6l Vwhich blanks out portions of the received signal during the occurrence of the synchronizing signal, as will be explained in greater detail hereinafter.

Briefly, synchronization of the receiver is accomplished by the reversal in polarity of the transmitted synchronizing pulses which swing the entire amplitude of the transmitted signal. lThe reversal in polarity of the synchronizing pulse triggers the third multivibrator Tl which, after a small adjustable delay provided by the phase control 99, in turn triggers the rst multivibrator I which sends a pulse down the receiver delay line for producing the receiver coding Wave K. The third multivibrator 'Il is completely insenstive to all incoming information until approximately 2 percent of a K Wave period before the scheduled arrival of a synchronizing signal. In the absence of the synchronizing signal the third multivibrator cannot trigger itself, but after an additional 5 per cent of said period it is triggered (if a synchronizing pulse does not occur), by a local pulse derived from the far end of the delay network. During this seven percent interval, the,

sensitivity, or susceptibility, of the third multivibrator increases only slightly, not being sumcient to trigger itself, `except after a much longer period than the' percent interval. The relatively low value of 2 percent of the pulse pe-` 7 riod for the preparatory time of the third multivibrator is made possible by the frequency stability of the receiver delay line.

Accordingly, it is evident that the synchronizing signal is effectively separated from other transmitted information, mainly by its occurrence and recurrence in time. The use of such a system necessitates that there should be momentary intervals occurring once during each pulse period during which only synchronizingY information is transmitted in order to allow pullin time for synchronism. Another condition is that whatever other information is transmitted must be sufficiently random, or harmonically unrelated to the synchronizing frequency, so that.

the receiver may not be synchronized on an incorrect pulse. The system described has been found to be quite satisfactory for the transmission of speech. The synchronizing signal information also is distinguishable from the re--fl mainder of the communication signal by its polarity and rate of change of magnitude. The third multivibrator is sensitive to transmitted pulses in only one polarity, and also the synchronizing signal is differentiated before it isV Asynchronizing circuits and by slight integration 'tiated and clipped. that after the occurrence of a synchronizing sigafterl the synchronizing pulse has been'diereIi-rA The integration requires Receiver synchronizing circuit In the receiver the blanking multivibrator 'l5 is triggered by pulses derived from the delay line in exactly the same manner as in the transmitter. The front edge of the blanking multivibrator pulse, shown in graph M of Figure 3, then triggers the third multivibrator Tl. The back edge of the pulse derived from the third multivibrator l?, in the receiving condition, does not occur automatically as in the transmitter. Instead the received and differentiated signal triggers the back edge 0f the third multivibrator pulse. The thus triggered third multivibrator pulse back edge then triggers the front edge of the pulse derived from the rst multivibrator l as in the transmitter. The back edge of the first multivibrator pulse then actuates the dierentiator amplifier 3 which in turn excites the delay line and propagates a succeeding pulse therealong.

The received synchronizing signal is applied, with its second half of positive polarity, to the input of an amplifier tube ll such that with l large amplitude signals the first half of the signal will be clipped, thus leavin-g a clean pulse edge at the beginning of the second half of the signal.

Referring t0 the circuit of Figure 4, the anode 'i circuit of tube lili includes a 1 megohrn series resistor W3. This high anode resistance attenuates high frequencies except for the pulse edges which drive the grid to zero potential, in which case the actual anode impedance of the tube shunts the 1 megohm anode resistor and improves the high frequency response. rEhe desired edge of the synchronizing signal applied to the grid of the tube is thus favored, while other signal edges are considerably integrated-out. The anode potential is then applied through a small capacitor i E5 to the grid of a second tube |07 which includes a 2.7 megohm grid leak IBB connected to a source of positive potential. This circuit comprises a non-linear differentiator. The desired signal edges, which are negative at this point, drive the grid of the second tube l'l sharply to a negative potential. The connection cf the grid leak |09 to the positive potential source discharges the small capacitor thus differentiating the pulse edge, and returning the grid of the second tube |01 to zero bias. Positive pulse edges, however, drive the grid of the tube further into conduction. The received synchronizing signal thus charges the capacitor |05 immediately by means of the grid current. Most of the signal voltage therefore disappears across the capacitor 105, only a very small portion appearing across the grid.

, rIhe Vanode Yof the second tube |01 is connected plin'g condenser H3 to the output of the blanka ing multivibrator 15.

Before a synchronizing signal is applied thereto, the blanking multivibrator has driven the anode circuit of the diierentiator tube |01 to a highly negative potential due to the connection thereto through the 390 micromicrofaradY capacitor 3. In this condition the tube |01 effectively has its anode below ground potential and can pass no applied signals. The anode resistor ||5 of the diferentiator tube |01 discharges the 390 micromicrofarad capacitor 3 and soon thereafter applies a positive potential to the anode of the tube |01. At this time, however,...due to the rst half of the synchronizing pulse, the grid of tube |01 is at zero potential, and l,plate current in the tubelimits the plate voltage to a very low value. The reversal in polarity of the synchronizing pulse drivesthe grid of Vtube |01 to a negative potential and the charging `oi" the 390 `micromicrofarad capacitor ||3 then continues.

The pulse applied to the grid of the lrst multivibrator tube of the third multivibrator 11 from `the blanking multivibrator 15 previously had also triggered the third multivibrator which at Athe time was sensitive to pulses in the negative direction. After this triggering of the third multivibrator it becomes sensitive to pulsesinthe positive direction. Therefore, when the synchronizing pulse permits the anode of tube |01 andthe grid of tube ||I to become positive, the third multivibrator 11 is triggered by the charging off the 390 micromicrofarad capacitor H3; -This condition forms the back edge of the l pulse provided by theY third multivibrator 11 which back edge in turn triggers the first multlto the grid of the tube l of the third multivibrator 11 is shown in graph V. l

The phase variation between the receiver de'- layline'pulse and the received svncliroruzing signal is accomplished by varying the Width of the'pulse derived from the rst multivibrator l. Thisfis accomplished by varying the resistance of thegrid leak of the first tube I i9 of the rst multivibrator l which is connected tothe source of` positive anode potential. The adjusted grid leak resistor then varies the time relation, or the delaybetween the received synchronizing pulse and the pulse which is applied to the receiver delayv line. Since the frequency of the receiving and transmitting systems is identical due to the use of substantially identical delay lines, the duration o f one complete cycle is necessarily substantially identical. Therefore, .if a delay is introducedinone portion vof the cycle, an,V equal amount of time must be subtracted from some re- 10 mainlng portion of the cycle in order to maintain the total of the cycle duration at a constant value. This is accomplished,automatically in that the third multivibrator 11 after being triggered by the blanking multivibrator 15 Will simply Wait until the synchronizing pulse arrives.

Additional security against noise signals may be obtained by varying the instant at which the third multivibrator may be triggered positively by the synchronizing pulse. This feature is essential, since noise signals and transmitted speech signals, which may immediately precede the synchronizing pulse, prevent proper synchronization if not excluded from the third multivibrator. The phasing control 99 preferably is a 2 megohm potentiometer having its sliding contact connected to the positive terminal of the power supply. One end of this potentiometer varies the Width of the pulses provided by the rst multivibrator, and the other end varies, in an opposite sense, the time from the front edge of the blanking signal beyond which time the third multivibrator may be triggered. The one time, or delay may thus be increased while the other time is simultaneously decreased, the two delays or 4intervals automatically adding to a constant total time. Thus the essential feature of the instant synchronizing system provides for a change from complete insensitivity to a substantially uniform sensitivity to synchronizing signals at an adjustable interval after the arrival of a preparatory timing or keying signal derived from the delay network. Also the improved method and system provides for treating the synchronizing signal such that pulses or noise in the incorrect polarity are substantially eliminated by differentiation While the synchronizing pulses in the correct polarity are utilized at full amplitude.

As explained heretofore with respect to the operation of the multivibrator circuits in the transmitting condition, if the circuit falls out of synchronism, the various multivibrators will provide pulses at somewhat increased time intervals until such time as a synchronizing pulse occurs at a proper instant to pull all of the multivibrators back into synchronism. Since pulses are derived from the delay network 2 at intervals of the order of .01 second, it is apparent that the various circuits will fall into synchronism in a relatively short time which seldom will exceed one full second.

Due to phase distortionin the transmission or radio circuits interconnecting the transmitter and receiver units, it is possible that the effective time of occurrence of the received synchronizing pulses, as indicated in graphs N, O and T of Fig. 3, will vary in different receivers with respect to the received coded speech. To correct for such variations, the circuit constants of the rst multivibrator and the third multivibrator 11 may, by means of the phase control 99, be altered in the receiving condition so that the width of the pulses W and Y may be varied to provide keying of the delay line 2 by the first multivibrator at the precise desired instant. The manner of varying the circuit constants of multivibrators to provide pulses of desired polarity and duration in response to predetermined applied keying pulses is well known in the art.

Signal decoding system The received signals derived from the radio receiver are applied to the input of the blanking circuit 61-1which, interrupts the received coded signals during the occurrences of the recurrent blanking pulses M, whereby the transmitted positive and negative synchronizing pulses maybe re-f Y moved from the received coded signal. This con- I dition obtains when the coding signal generator of the receiver is in synchronism with vthe transmitter coding signal generator, since the blanking multivibrator is responsive to pulses derived from the last tap on the delay network 2.

12 cuit,` the Lwaves S and K, to be multiplied, are added together with four ydiierent polarity combinations and squared in four different signal channels. Then the four squared signals are added together with suitable polarities to obtain the product SK Vin the output circuit of the multiplier network, as will be illustrated by the fol--V lowing equations:

Blanking circuits are well known in the art. They may comprise, for example, a push-pull amplifier for the signal, arranged so that the blanking pulses M are superimposed on the gridcathode circuits so that both tubes are simultaneously driven to cut-off duringV the blanking pedescribed in detail hereinafter by reference toV Figure 6 of the drawings. Signals derived from the reciprocal circuit 3| will have instantaneous` ordinates corresponding Vto the reciprocal values of the instantaneous ordinates of the synchronized coding wave K generated in the receiver. The reciprocal wave |/K is applied through the rst transmit-receive switch 5| to a second input circuit of the multiplier 6|.

Since the wave multiplier 3| provides output signals which have instantaneous ordinates corresponding to the product of the instantaneous ordinates of the waves |/KY and SK applied thereto, the output signals applied through the third transmit-receive switch 63 to a reproducer |33 will be substantially characteristic of the original speech modulation signals S. The signals applied to the reproducer |33 have been indicated as S since some phase distortion is inherent in the various circuits described and especially in many radio transmission circuits. It should be understood that the signals S derived from the third transmit-receive switch 53 may be applied to actuate any other desired type of utilization apparatus, not shown.

Signal multiplier Figure 5 shows a typical wave multiplier circuit forming a portion of both the coding wave transmitter and receiver system circuits described heretofore with reference to Figure 1 of the drawing. This multiplier circuit is described and claimed in the copending U. S. application of Alda V. Bedford, Serial No. 517,967, led January l2, 1944, now U. S. Patent No. 2,401,404, and

It will be understood that. the term a in the.

above equations is the D.C. bias added to the A.C. waves to cause all of the signal amplitude variations to vhave the same polarity with revspect to the squaring devices.

assigned to the same assignee as the instant appli- Y The squaring circuit illustrated employs a plurality of small copper oxide` rectiers known commercially as Varistors Because of the particular variable resistance characteristics of `the Varistorf the Vcurrent therethrough is substantially proportional to the square of the applied voltage over a reasonable range of applied voltage of a single polarity. The Vmultiplier network 6| is shown as including a rst triode thermionic tube |35 having its grid electrode connected to the movable ,contact of the first transmit-receive" switch 5|, whereby signals characteristic of either the coding wave K or the reciprocal thereof I/K may be applied to the tube grid-cathode circuit. ,n

A second thermionic tube |31 has its grid electrode connected to the movable contact of the second transmit-receive switch 59, whereby either the speech signals `S or the blanked, received signals SK may be applied to the tube grid-cathode circuit. The operation of the circuitl will be explain-ed hereinafter with the switches 5| and 5 9 in the transmitting position whereby the signals K and S, respectively, are

' applied to the grid-cathode circuits of the tubes |35 and |31. Push-pull output signals are derived from each of the tubes by means ofl connections to the corresponding tube anode and cathode circuits as indicated in the drawing.

In orderv that the desired sum voltages be obtained, the signals S and K are applied to a network of resistors in the following manner: Signals S and K respectively traverse resistors |39 and |4| to provide a signal proportional to (S-I-K) at point (S-l-K); the signals S and -K respectively tr-averse resistors |43 and |45 to provide signal (S-K); the signals v-S and -K respectively traverse resistors |41 and |49 to provide signal (-SK) and the signals -S and K traverse respectively resistors |5I and |53 to provide signal (-S{K). Thus,'at each of the four junction points, a sum of voltage is obtained as designated in the circuit diagram. As shown, the network also includes resistors |55 and |51 leading respectively from points (-S-K) and S-l-K) to ground, and resistors |59 and |6| leading respectively from points Y(S-l-K) and S-K) to the positive terminal of the source of bias voltage which is applied through a voltage-reducing resistor |63. An SOOO-ohm resistance has been found satisfactory for the resistors |55, |51, |59 land |6| while 100,000-ohm resistance has been taken as the value of resistors |39, |f4|, |43, |45, |41, |49, |5| and |53.

awa-csa;

Thesum voltages at the four'points of the network are applied with bias voltages a or -a to four Varistors CO1, CO2, CO3, and CO4, respectively, all of which control the current through the common load resistor |65 to provide thereacross the product output voltage SK. The output across |65 is proportional to the sum of all the voltages which would have been generated if each Varistor had supplied current to a separate resistor, as indicated by the foregoing squaring equations. It is to be noted that the Varistors CO2 and CO4 are connected with opposite polarities from the Varistors CO1 and CO3, so that the D.-C. bias voltage must b different.Y Y

By reference respectively to the third and fourth equations it will be seen that the values (-S-l-Ka) and (S-K-a) are each preceded by another minus sign and included in brackets before squaring to` indicate properly mathematically the effect of the reversed connection on these two Varistors. These rive equations show that, ideally, only the desired voltage SK is produced across the output resistor |65.

`For compensating for small dissimilarities in the Varistors and other circuit elements, it has been found desirable to provide variable resistors |61 and |99 connected as voltage dividers in the anode circuits of the tubes |35 and |31, respectively, for adjusting the relative amplitudes of -S and -K.

While in the foregoing the term multiplying circuit has been used to define the circuit, it will be seen thatthe circuit actually is a sort of modulator which is completely balanced in the sense that only the side band frequencies are produced,` while the input frequencies and the harmonics thereof are suppressed.

The output signals SK derived from across the output resistor |95 are applied to the movable contact of the third transmit-receive switch 69, whereby they may be selectively applied to either the reproducer |33 or to the blanking circuit 91, depending upon the desired operation of the circuit of Fig. 1.

`Signal reciprocal circuit The reciprocal circuit |3| shown in Figure 6 of the drawing is described and claimed in the copending application of Carl A. Meneley, Serial No. 484,304, filed April 23, 1943, now U. S. Patent No. 2,403,549, and assigned'to the same assignee as the instant application. In this circuit instantaneous reciprocal values of an applied coding wave K are obtained by means of an electrical network in which the wave K is clipped on both its positive cycle and on its negative cycle to produce a substantially rectangular wave, and in which the wave K and the rectangular waive are added together with one of them reversed in polarity, preferably after the peaks of the positive and negative cycles of the wave K have been squashed or attened somewhat. The circuit includes no appreciable capacitive or inductive reactances (the blocking capacitors in the circuit presenting negligible impedance) and, therefore, provides the reciprocal of substantially any applied signal waveform regardlessof its frequency components.

Referring to Figure 6, the graph b represents a typical coding wave K which is applied to the input terminals |1| of the circuit. The graph e represents the reciprocal wave |/K, which isthe sum of the iiattened wave K, represented bythe graph c, of reversed polarity, and of -the rectanguiar wave shown in graph d. The squashed or lattened wave c may be obtained by passing the wave b through a circuit that changes its resistance with a change in applied voltage. The rectangular wave d may be produced by clipping the positive and negative cycles of the wave c at the voltage levels f and y respectively, for example, close to the A.C. axis of the signal, and then by amplifying the clipped signal.

VThe wave K applied to the input terminals |1| may?, if desired, be amplified by means of an ampliiier tube |13 to provide a peak-to-peak amplitilde, for example, of the order of 60 volts. The amplified K wave then is applied through a blocking capacitor |55 and a resistor |11 to a copperoxide rectifier unit |19 which functions as a nonlinear resistor having the property of decreasing in resistance as the applied voltage increases. The resistor |11 is cf high enough resistance so that the driving source for the non-linear resistance unit |19 is of high impedance whereby there is only a slight variation in the current ow through the unit |19. The unit |19 may consist of a pair of copper-oxide rectiers |8| and |83 connected to conduct current in opposite directions.

The voltage appearing across the non-linear unit |19 is the voltage wave c, which is the Wave K having a flattened waveform. This voltage, which is amplied by a cathode-biased vacuum tube |85, appears across an anode resistor |81 and a portion of a second anode resistor |89 of a second amplier tube |9I.

The rectangular wave d is produced, in this ,particular example, by applying the output of the tube through a blocking capacitor 93 and a high resistor to a pair of diodes |91 and |99, which are connected to conduct in opposite directions. Resistors 20| and 293, of comparatively low resistance, are connected in series with the diodes |91 and |99, respectively. A biasing voltage drop for opposing current ow through diode |99 is produced across the resistor 293 by connecting a source of voltage (not shown) thereacross, a resistor 295 being in series with the voltage source. The diodes |91and |99 clip the applied wave c symmetrically about its A.-C. axis, because a voltage which causes current flow through the diode |91 and resistor 29| is built up across the capacitor |93 by the positive cycle pulses flowing through the diode |99. Thus, the diodes |91 and |99 become conducting on alternate half cycles when the signal voltage exceeds the D.-C. voltage drop across the resistors 29| and 293, respectively. The resulting rectangular wave d is amplied and reversed in polarity by the tube 9|. The wave c and the ilattened wave d add in the portion of the second anode resistor |89 that is common to the tubes |85 and |9i to produce the desired reciprocal wave I /K shown in graph e.

If the wave c is attened correctly, and if the waves c and d are added with the correct relative amplitudes, the resulting signal will be substantially a true reciprocal of the wave K. The only substantial departure from a true reciprocal signal will be where the K wave crosses the A.C. axis. Here the reciprocal value is innity whereas the maximum amplitude of the wave /K necessarily has a nite limit. The waves c and d may be mixed with the correct relative amplitudes byf adjusting a variable tap 291 on the second anode resistor |89. The correct shaping of the flattened wave c may be obtained by selecting a non-linear resistor unit |19 having a suitable.

voltage-resistance characteristic and by adjusting the value of the variable resistor ill.

As previously noted, the above-described reciprocal circuit is purely resistive so that its operation is independent of frequency. The instantaneous voltage output of the circuit is always substantially the reciprocal of the instantaneous applied voltage. It follows that if the reciprocal circuit is adjusted to produce the reciprocal of an applied signal having one wave form, the circuit will then always produce the reciprocal of an applied signal regardless of its wave form. There are various ways of determining when the circuit has been adjusted to give substantially a true reciprocal. One Way is to connect the reciprocal circuit into the signalling system of Fig. 1 and, while transmitting speech or music, adjust the resistor l'l'l andY the variable tap Ztl at the receiver until the speech or music has a minimum of distortion.

It should be understood that oppositely-connected diodes may be substituted for the copperoxide rectiiiers ISI and 83 described heretofore. When properly biased, the two diodes should be operated along the lower knee of their operating characteristic and in the proper region to shape the K wave in the desired manner to provide the wave c.

It will be understood that the invention is not limited to the particular circuits illustrated since the waves c and d may be derived from the K wave in various other ways, and since the two waves may be combined by means of a variety of other circuits.

Thus the invention disclosed comprises an improved method of and means for synchronizing in an extremely precise manner a plurality of wave generators of any types which include a pulse driving source. The preferred embodiment of the invention is described herein in combination with a secret telecommunication system wherein the invention is employed to synchronize coding and decoding wave generators at the transmitter and receiver, respectively, and wherein positive synchronization throughout an adjustable time interval is accomplished by utilizing substantially only synchronizing pulses of one predetermined rate of change of polarity to initiate synchronization of the receiver after an extremely short predetermined interval of receiver sensitivity to synchronizing information.

I claim as my invention:

1. A synchronizing system for two devices including synchronizing generating means responsive to one of said devices for generating discontinuously and recurrently a nrst signal pulse immediately followed by a second signal pulse of opposite polarity than the said first pulse, means for deriving synchronizing control signals in response to the reversal in polarity between said rst and second pulses, means for applying said control signals to synchronize the other of said devices with said one of said devices, and means for varying recurrently at the frequency of said synchronizng control signal intervals the sensitivity to signal energy of the other of said devices whereby said other of said devices is responsive to said control signal energy during only a predetermined portion of said signal interval.

2. A synchronizing system for two devices including synchronizing generating means responsive to one of said devices for generating discontinuously and recurrently a first signal pulse immediately followed by a second signal pulse of opposite polarity than the said first pulse, signal differentiating'means for combining said pulses with signals from said one device, means for deriving synchronizing control signals in response to the reversal in polarity between said rst and second pulse components of said combined pulses and signals, means for applying said control signals to synchronize the other of said devices with said one of said devices, and means for varying recurrently at the frequency of said synchronizing control signal intervals the sensitivity tosignal energy of the other of said devices whereby said other of said devices is responsive to said control signal energy during only a predetermined portion of said signal interval.

.3. A communication system including a plurality of signal devices, means for providing a synchronizing signal comprising recurrent groups of pulses each consisting ofY a Jfirst signal pulse immediately followed by a second signal pulse of opposite polarity than said first pulse. means for blanklng signals from one of said devices during intervals including each of said pulse groups, means for combining said blanked and said synchronizing signals, means for synchronizing another of said devices with said one of said devices comprising means for deriving control pulses in response to the reversal in polarity between said rst and second pulse signal components of said combined signals, a pair of keying devices, means for applying said derived control pulses to actuate one of said keying devices, means responsive to said actuation of said one keying device to actuate the other of said keying devices, means for varying recurrently at the frequency of said control pulses the sensitivity of said one keying device to said control pulse signal energy whereby said one keying device is responsive to said control pulses during only a predetermined portion of said pulse signal intervals, and means responsive to actuation of said other of said keying devices to actuate another of said signal devices in synchronism with said one of said signal devices.

4. A communication system including a plurality of signal devices, means for providing a synchronizing signal comprising recurrent groups of pulses each consisting cf a rst signal pulse immediately followed by a second signal pulse vof opposite polarity than said rst pulse, means for blanking signals from one of said devices during intervals including each of said pulse groups, means for Vcombining said blanked and said synchronizing signals, means for synchronizing another of said devices with said one of said devices comprising means for deriving control pulses in response to the reversal in polarity between said iirst and second pulse signal components of said combined signals, a pair of multivibrator switching devices, means for applying said derived control pulses lto actuate one of said switching devices, means responsive to said actuation of said one switching device and to signals from another of said signal devices to actuate the other of said switching devices, means for varying recurrently at the frequency oi said control pulses the sensitivity of said one switching device to said control pulse signal energy whereby said one switching device is responsive to said control pulses during only a predetermined portion of said pulse signal intervals, and means responsive to actuation of said other of said switching devices to actuate said other of said signal devices in synchronsm with said one of said signal devices.

5. A communication system including a plurality of signal devices, a synchronizing signal generator for synchronizing said devices including means for deriving signals from a first of said devices, means responsive to said rst device for generating discontinuously a rst signal pulse, means responsive .to said rst pulse means for generating a second signal pulse of opposite polarity than and immediately following said iirst pulse, means for combining said derived signal and said synchronizing signal pulses, means for deriving a carrier signal, and means for modulating said carrier signal by said combined signals in a manner whereby said reversal in polarity of said synchronizing pulses drives said carrier signal substantially to zero amplitude.

6. A system according to claim including means for blanking said derived signal during each interval of said synchronizing signal.

7. In an improved synchronizing system for two signal devices, one of said devices providing a source of modulated signals including a discontinuous synchronizing signal comprising a rst signal pulse immediately followed by a second signal pulse of opposite polarity than said rst pulse, the reversal in polarity between said pulses substantially reducing the amplitude of said modulated signal to a zero Value, the improvement comprising a signal receiver responsive to said modulated signal source and including means for deriving synchronizing control signals in response to the reversal in polarity between said rst and second pulse components of said modulated signals and means for controlling recurrently at the frequency of said control pulses the sensitivity of the other of said devices to said control signals whereby said other device is responsive to said control signals during only a predetermined portion of said pulse signal intervals.

8. A communication system including a plurality of signal devices, a synchronizing signal generator for synchronizing said devices including means for deriving communication and recurrent discontinuous synchronizing signals from a rst of said devices, means for combining said derived communication signals and said syn# chronizing signals, means responsive to said synchronizing signal component of said combined signals for deriving a control signal, a pair of keying devices, means responsive to another of said signal devices for energizing one of said keying devices only during recurrent short time intervals corresponding to the sequence and rate of said synchronizing signals, means responsive to said energization of said one keying device and to said control signal for actuating said other of said keying devices, and means responsive to 18 actuation of said other keying device for synchronizing said other signal device with said one signal device.

9. Apparatus according to claim 8 including means for controlling the operating intervals of said keying devices.

10. Apparatus according to claim 8 including means for varying lthe timing of said keying devices with respect to the timing of said synchronizing signals.

11. Apparatus according to claim 8 including means for blanking said communication signal component of said combined signals substantially during the actuation intervals of said keying devices.

12. Apparatus according to claim 8 wherein said keying devices are responsive .to said combined signals during only a predetermined small portion of said synchronizing signal intervals.

13. A communication system including a plurality of signal devices, means for generating recurrent discontinuous synchronizing signals, means for combining said synchronizing signal with signals from one of said devices, means for transmitting said combined signals, means for receiving and segregating said synchronizing signals from said transmitted combined signals, means combined with said receiving means for generating second synchronizing signals, means for synchronizing said iirst and said second synchronizing signals, and means for controlling recurrently at the frequency of said synchronizing signals the sensitivity of said second synchronizing signal means to said received synchronizing signals whereby said second device is responsive to synchronizing signals during only a predetermined portion of the synchronizing signal periods.

KARL R. WENDT.

REFERENCES CITED The following references are of record in the le of this patent:

UNITED STATES PATENTS Number Name Date 2,141,343 Campbell Dec. 27, 1938 2,231,792 Bingley Feb. 11, 1941 2,258,943 Bedford Oct. 14, 1941 2,277,000 Bingley Mar. 17, 1942 2,350,536 Schlesinger June 6, 1944 2,359,447 Seeley Oct. 3, 1944 2,401,405 Bedford June 6, 1946 2,406,977 Wendt Sept. 3, 1946 2,406,978 Wendt et al. Sept. 3, 1946 2,408,077 Labin Sept. 24, 1946 

